Regulation of power converters is typically accomplished by pulse width modulation achieved by controlling a duty cycle of the power switch or switches in response to a regulatory error signal. At low duty cycles the power switches conduct for a very short interval of the cycle. Due to the narrow current pulse the output circuit of the inverter requires a sizable inductor and capacitor to provide the energy storage requirements necessary to produce a substantially ripple free DC output signal.
If the power converter is powered from an AC power line it includes an input circuit that rectifies the AC voltage and filters it to supply a DC voltage at the input to the inverter section of the converter. This arrangement also typically requires an input rectifier with accompanying sizable inductive and capacitive components. Normally a charge storage capacitor is connected across the rectifier output. This capacitor quickly charges up to a DC voltage approximating the average value of the input AC voltage. This DC voltage backbiases the diodes of the rectifier during most of each half cycle of the applied sinusoidal voltage and hence the interval for conduction of the rectifier diodes in each half cycle of operation is very short. Short current spikes with a narrow conduction angle during the sinusoidal half cycles are generated and result in a poor power factor at the input to the rectifier.
This poor power factor at the input may be partially compensated for by inserting a low frequency inductor between the rectifier and the filter capacitor. This arrangement is generally not suitable if size of the converter is critical or if cost must be kept low because of the size of the inductor required.
Another technique to compensate for the poor power factor is to insert the inductor within the converter power train between the rectifier and the charge storage capacitor while varying the power switch duty cycle to obtain regulation. Here the switching frequency of the converter dictates the size of the inductor, however it may still be a sizable portion of the circuit package. A much higher frequency of operation often permits the use of a physically smaller inductor. However this arrangement imposes large stresses on the power switching devices of the converter during the on-off and off-on power switching transitions.
Active control of the current wave shape has also been used to control the input impedance to approximate a resistive load at the input and produce a very high power factor at the input. In one arrangement a boost type converter with a feedback control to control the duty ratio of its power switch is inserted between the input rectifier and the storage capacitor to control the current waveform so that it is substantially similar in waveform and in phase with the input sinusoidal voltage. A subsequent converter is used to achieve line and load regulation. This widely used arrangement however requires added circuitry that adds complexity to the overall power train and is operative solely to enhance the input power factor while complicating the overall power converter operation.